LONDON — When the United States declared war on Al Qaeda after the Sept. 11 attacks, American leaders took the fight to the militant group’s hide-outs in Afghanistan, a faraway and failing state, with an invasion and occupation.

But for Europe’s leaders, who now consider themselves at war with the Islamic State after large-scale terrorist attacks at home, the challenge is more complicated: The enemy’s hide-outs are ghettoized parts of Paris, Brussels and other European cities that amount to mini failed states inside their own borders.

While France and Britain have joined the United States in bombing Islamic State targets in the Syrian city of Raqqa and other areas controlled by the group, also known as ISIS and ISIL, Europe has faced a much harder time understanding and dealing with its own citizens who have abetted the Islamic State’s ascent. These are mostly third-generation Muslim immigrants, who have become radicalized in poor communities left to develop outside the national culture.

Those communities are incubators that figure prominently in the Islamic State’s two attacks on Paris since January 2015, and the bombing on Tuesday in Brussels.

Resolving the problem, political analysts say, does not require simply more intelligence cooperation and shared lists of people suspected of being radicals and fighters returning from Syria. European governments must also develop internal strategies to deal with the threat at home — the deep social problems of racism and radicalism, along with the security dilemma, which raises concerns about surveillance, justice and civil liberties.

“You can bomb Raqqa, and you may consider that to be war, but you’re not going to bomb Molenbeek or Schaerbeek or St.-Denis, unless you’re ready for civil war,” said François Heisbourg, president of the International Institute for Strategic Studies, referring to heavily Muslim areas of Brussels and Paris.

President François Hollande of France and his prime minister, Manuel Valls, have repeatedly described Europe’s fight against the Islamic State as war; Mr. Heisbourg called that an “extremely dangerous” use of the word. “It starts with Raqqa and can end up with the Algerian civil war, and that would be the ultimate victory of Daesh,” he said, using an Arabic term for the Islamic State. “They want to divide our societies against ourselves.”

Even more, Mr. Heisbourg argued, “talking of war dignifies Daesh, which wants to be seen as having a state and an army of warriors and martyrs.” For angry, poor and isolated young Muslims in Europe, “to be seen as the downtrodden victims of Western colonialism and iniquity, fighting the holy war against the arrayed legions of the crusaders,” is precisely what the Islamic State advertises.

The dual nature of the European struggle against the Islamic State separates it from the American “global war on terrorism” and deeply complicates it, argued Raffaello Pantucci, director of international security studies at the Royal United Services Institute, a defense-oriented research institution.

“We need a dual response,” he said, more bellicose on the Islamic State abroad but less so at home, emphasizing longer-term social work in isolated and disenfranchised communities.

The objective, he said, is to counter radical voices who often provide paths into meaning for young men who have been petty criminals. Most of the European terrorist suspects were known to the local police.

“There is a realization that this is not a war you can bomb or shoot your way out of, but you have to deal with individuals who are radicalized at home, to examine the reasons that they are exploring this other identity,” said Mr. Pantucci, who wrote a book on the issue: “We Love Death as You Love Life: Britain’s Suburban Terrorists.”

Important pockets of the disenfranchised and isolated are embedded in most European countries, he said: Bradford in England, heavily Kashmiri and home to the London subway bombers of July 7, 2005; largely Muslim East Birmingham, where organized crime and radicalism spring from the same roots; and the heavily immigrant suburbs or banlieues of France’s big cities.

Belgium, already divided by language and with a plethora of local and state federalisms and police forces, provides a special example. More so than elsewhere, Belgium allowed the self-ghettoization, or self-isolation, of ethnic communities in the name of multiculturalism and peace.

“There are parts of Europe, especially in France and Belgium, where over the past two decades you’ve seen the emergence of essentially ungoverned spaces, nearly akin to Yemen or Libya,” said Peter R. Neumann, director of the International Center for the Study of Radicalization and Political Violence at King’s College London. “Molenbeek is one of them, a place where local authorities and even mainstream Muslim groups abandoned them, with an informal pact, that ‘as long as we don’t see you, we won’t bother you.’ ”

Criminal groups, but also Islamist radicals, soon “figured out that this local anger could be channeled into radical extremism,” he said. Resentment and alienation from the state meant that these groups “could enter and work without being hassled by the police but also found people open to their message.”

Mr. Heisbourg said that the result was “a disaster for counterterrorism.” These were “no-go areas for the authorities, who have found it very difficult to get informants and human intelligence,” noting that many of the French citizens who carried out attacks in France lived or were hosted in Brussels neighborhoods like Molenbeek.

In per capita terms, more Belgians have left to fight with Islamic militants in Syria and Iraq than citizens of any other European country: As of last month, 441 had done so, according to data from the Belgian Ministry of Justice, and 117 have returned.

Places like Molenbeek and Schaerbeek, where the bombs used on Tuesday were thought to have been constructed, have been problems for a long time, Mr. Pantucci said. “One question is whether more could have been or should have been done to understand the local problems and deal with them. Because left alone, over time there is an undercurrent of radical ideologies,” radical preachers and recruitment networks, “and getting rid of that is very, very difficult.”

Some political scientists, like Olivier Roy, a scholar of Islam at the European University Institute, say that Islam does not cause radicalization, but serves as the vehicle for radicalized anger from some Muslim youth.

For that, Europe has few easy answers, especially as the Islamic State seeks to manipulate European fears of terrorism and migration. Gilles Kepel, a French sociologist who has studied radical Islam and the banlieues, has argued that part of the Islamic State’s intention is to mobilize fears of the “enemy within,” create further rejection of European Muslim citizens and radicalize them at home, to create a kind of civil war between European Muslims and the “crusader states.”

In dealing with terrorism through denial of nationality, abuses of civil liberties or an indefinite state of emergency, Mr. Heisbourg warned that “you may create the conditions where you end up with the civil war in European societies that Daesh clearly wants.”

A version of this article appears in print on , on Page A10 of the New York edition with the headline: Europe Faces Challenge of Fighting ISIS Within Its Borders . Order Reprints | Today’s Paper | Subscribe